PC users are being spoiled rotten this year: another piece of malware purporting to be a message from Santa is delivering unwelcome surprise gifts disguised in an electronic greetings card.

The Trojan horse 'MerryX.A' is delivered in an email, which encourages the recipient to open an attached "animation". While the animation of Santa Claus delivering presents plays, a piece of malware hiding behind the name 'SQLServer.exe' is installed on PCs running Windows, according to security staff at Panda Software SL's PandaLabs.

The software transmits information about the infected computer to a remote server, and then attempts to download files, which could include other malware, Panda said.

Earlier this week, security researchers identified an instant-messaging (IM) worm, IM.GiftCom.All. Once it has infected a computer, the worm searches the contact databases of installed IM applications, and sends messages to the contacts it finds, encouraging them to visit a website. If the recipients click on the link, the website attempts to download another piece of malware to infect their PCs, which in turn spread the message further.

The festive season provides plenty of opportunities for malware writers to hone their social-engineering skills, as internet users are unsurprised to receive brief messages with a seasonal theme from long-lost friends or distant business contacts.

This time last year, someone modified the Zafi email worm to spread itself in a message entitled "Merry Christmas".

As at any other time of year, security researchers advise users that the best defence is to check out unsolicited attachments or web links with the purported sender before opening or clicking on them.